1. Field of the Invention
The present invention relates to a direct methanol fuel cell system (DMFC), and, more particularly, to a capacitive liquid level detector for a DMFC.
2. Description of the Related Art
A fuel cell is a galvanic cell for converting the chemical reaction energy of continuously fed fuel and oxidant into electric energy. In general, a fuel cell includes two electrodes (or anode and cathode) separated by a membrane or by an electrolyte. The anode is surrounded by a flow of fuel, e.g., hydrogen, methane or methanol; and the fuel is oxidized there. The cathode is surrounded by a flow of oxidant, e.g., oxygen or hydrogen peroxide, which is reduced at this electrode. Depending on the type of fuel cell, the materials used to realize these components are to be selected differently.
Compact direct methanol fuel cells (DMFC) systems are currently in the focus of development in many electronics companies. They are expected to replace or append the power supply of mobile electronics devices because they allow longer operating times and a quicker recharge. A direct methanol fuel cell is a low-temperature fuel cell which is operative in a low temperature range of from about 60 to about 120° C. This type of cell utilizes a polymer membrane as electrolyte. Methanol (CH3OH), with no previous reforming, is supplied directly to the anode together with water to be oxidized there. Carbon dioxide (CO2) is formed as waste gas at the anode. Atmospheric oxygen supplied to the cathode as oxidant reacts with H+ ions and electrons to form water. The advantage of the DMFC lies in the use of a liquid, easy-to-store and very inexpensive source of energy, which can be distributed in plastic cartridges, for example. Moreover, a vastly branched infrastructure for methanol is already existing in many fields, e.g., through the use as anti-freeze additive in windshield washer fluids for motor vehicles. Depending on the design, this type of fuel cell can provide power ranging from some mW up to several 100 kW. More specifically, DMFCs are suitable for portable use as substitutes and supplements for conventional accumulators in electronic devices. Typical fields of use are in telecommunication and power supply of notebooks.
The oxidation of methanol on the catalyst of the anode proceeds step by step, and several reaction pathways with various intermediate products are being discussed. To maintain high efficiency of the fuel cell, rapid removal of the reaction products from the region around the electrode is required. As a result of the temperatures being encountered and the chemistry that constitutes the basis, a liquid/gas mixture of CO2, water, water vapor and non-reacted methanol is formed. Water and methanol should be recovered from this liquid/gas mixture so as to maintain self-sufficiency of the system for as long as possible. Furthermore, CO2 must be removed from the equilibrium. This is done by utilizing a CO2 separator. The CO2 should be removed from this liquid/gas mixture in order to re-feed the liquid fuel mixture to the anode after adjusting the methanol concentration. Separation of the gases is effected by utilizing a CO2 separator.
Similarly, a liquid/gas mixture is formed at the cathode, including non-consumed air, water and water vapor. To achieve long-lasting self-sufficiency of the system, as much water as possible must be separated from the liquid/gas mixture and the liquid/gas mixture from which water is separated must be re-fed into the cathode cycle. To this end, a heat exchanger is arranged downstream of the cathode outlet of the fuel cell so as to cool the mixture and achieve condensation of the water vapor.
Arranged downstream of the heat exchanger is an air separator separating the air stream from liquid water so as to re-feed the water into the anode cycle. Accordingly, the separators are mainly used in water management and to remove CO2 from the equilibrium. Conventional separators separate the phase mixture of liquid and gaseous or vaporous components and release the gaseous or vaporous components into the environment.
Amongst other things, a liquid storage tank is necessary for a stable system operation. The liquid level inside this tank should be known to (or determined by) the system in order to control the amount of water to be recycled.
A continuous level measurement is very beneficial for the system control as it renders possible to implement common proportional-integral-derivative (PID) control algorithms into the control software allowing a faster and better controlled reaction on changes in the system.
Liquid level measuring in small receptacles is a difficult matter. Common problems are the reliability and accuracy of the measured signal. Most of the established measuring principles are only usable in more or less large tanks.
Common capacitive measuring principles use the liquid as the dielectric between the capacitor plates. Some inventions incorporate a set of plates which dive (or are submerged) into the liquid, in other examples the plates are arranged at the exterior of receptacles.
As an example, a liquid level sensor is disclosed in U.S. Pat. No. 5,182,545. As a liquid rises and falls in the container, the dielectric effect of the liquid changes the effective capacitance of a sensing capacitor which is detected by electronic circuitry coupled to the sensor. One plate of the sensor capacitor is a probe disposed within a receptacle while a grounded conductive portion of the receptacle is a second plate of the capacitor.
A non-intrusive fluid level detector including a single point capacitive sensor mounted on the outside surface of a receptacle is disclosed in U.S. Pat. No. 5,017,909. A non-conductive container or a non-conductive window in a conductive container is used to place the sensor having insulated plates which are not in direct contact with the liquid. The sensor assembly is disposed on the exterior wall of the receptacle.
Both, U.S. Pat. No. 5,182,545 and U.S. Pat. No. 5,017,909 employ the fluid as the dielectric of the capacitor. However, since the capacitance of a plate capacitor is inversely-proportional to the distance between the plates, the use of the fluid as the dielectric and/or the placement of the plates on the exterior of the plates leads to a large plate distance and thus such a sensor leads to the disadvantage that the capacity and the capacity variation to be measured are very small. The small capacity results from the small plate area and the relatively large distance between them. Relatively expensive measuring electronics are required, and the measured signal is relatively inaccurate and also subject to influences from outer electrical fields.
U.S. Pat. No. 6,943,566 discloses a sensor applied to a wall of a container or integrated into the container. The wall of the container made of plastic or glass-fiber constitutes the dielectric, and the conductive fluid itself constitutes the second plate, i.e., the capacitor consists of a sensor plate and the contents of the container. However, because the wall made of plastic or glass-fiber forms the dielectric and the wall needs to be thick enough to contain the liquid inside, i.e., at least several millimeters, the distance between the plates is still significant and the capacitance is still low. Furthermore, no metallic parts are allowed to be present between the sensor plate and the fluid, because otherwise these metallic parts have an effect on the capacitance of the system. The precision and linearity of the measurement depends on the thickness and the uniformity of the dielectric constant of the wall material over the entire area of the sensor plate. The latter is difficult to achieve with common manufacturing methods of plastics.